Such devices can be used wherever the body core temperature is to be determined, but inaccuracies due to conventional peripheral measuring methods are undesirable or cannot be tolerated. Devices according to the present invention are especially suitable for monitoring firemen and rescue personnel, who are exposed to physical strains at extreme temperatures. Strain limits can be set and mission decisions can be made with the data thus obtained.
Another field of use is represented by patient monitoring in medical applications. In particular, the monitoring of diseases associated with fever and patient monitoring after severe hypothermia can be effectively accomplished by monitoring the body core temperature.
Measuring or monitoring the body temperature as an important vital parameter has been known for a long time. The body core temperature is a highly informative parameter concerning vital functions of a subject. However, it cannot be readily determined by conventional temperature measuring methods. Temperature measurements at more or less peripheral measuring points show varying deviations of the measured temperature from the body core temperature. These deviations are either difficult to calculate, lead to poorly reproducible results or show highly sensitive dependences on various extraneous effects.
Various efforts have been made to reduce these effects by clever measuring set-ups. Thus, it is known that the temperature can be measured within the armpit or rectally in the case of the measurement of fever. What is most important in connection with the measurement in the armpit is to position the thermometer in a stable manner with the arm held in a constant position. This considerably restricts the freedom of movement, and this method is therefore inherently ruled out for the monitoring of active mission personnel. Rectal measurement may be dangerous when full freedom of movement is required at the same time. Even though the above-mentioned two possibilities represent a step towards measurement near the body, the measurement is still carried out at a relatively great distance from the core. In addition, both are ruled out for the monitoring of active mission personnel due to the restrictions associated with them in terms of the freedom of movement.
It is known that temperature sensors can be arranged in the armpit by means of holding mechanisms, which guarantees the freedom of movement of the arm (U.S. Pat. No. 4,747,413). However, these arrangements are suitable for monitoring fever only and have the drawback that the body temperature is measured at a relatively great distance from the core. It is known, furthermore, that the surface temperature of humans can be measured or monitored by placing various temperature sensors on the body surface (GB 2 309 304 A). However, this method likewise fails to open up a possibility of measuring the body core temperature. It is known that sensors used for temperature measurement can be arranged near the surface and that these sensors can be combined with sensors for monitoring other vital parameters (DE 199 27 686 A1). It is also known that temperature sensors arranged near the surface can be combined with means for determining the position of the wearer of these sensors (DE 100 05 526 A1). Integration of individual sensors in articles of clothing was proposed as well (DE 199 27 686 A1).
All these measuring set-ups are characterized by the shortcomings of a near-surface temperature measurement that were already mentioned above.
It is, furthermore, known that the heat flow through the body surface can be measured and the body core temperature can be inferred from this. Devices with double temperature sensors are especially suitable for measuring the heat flow through the body surface. Double temperature sensors in the sense of the present invention comprise essentially two temperature sensors, which are arranged at a fixed distance from one another in a holding construction such that one temperature sensor assumes a position near the body and is in contact with the skin, the second temperature sensor is arranged at a defined distance from the first temperature sensor in a position away from the body, and a defined heat flow takes place between the two sensors.
A very readily predictable, known or reproducible sensor thermal conductivity can be obtained for the material/medium in the space between the sensors due to a special embodiment of the holding construction. The thermal conductivity of a particular piece of material/medium is determined by the size and shape of the particular piece, and the specific thermal conductivity unit value or thermal conductivity constant for that specific material. When the particular piece of material/medium is the standard or size of one unit, the thermal conductivity is by definition equal to the thermal conductivity unit value or thermal conductivity constant for that specific material. The two values are often used interchangeably. If, in addition, the body tissue thermal conductivity of the tissue is determined, such as by estimation, between the body surface at the site of the sensor and the zone in which nearly the body core temperature prevails, the body core temperature can be extrapolated from the sensor and body tissue thermal conductivity, i.e., the estimated body tissue thermal conductivity of the tissue and the known sensor thermal conductivity of the material/medium between the double temperature sensor. The formulaTcore−T1+(T1−T2)/ks/kt in which    Tcore is the body core temperature    T1 is the temperature in at a position near the body    T2 is the temperature in at a position away from the body    Ks is the known sensor thermal conductivity of the material/medium between the double temperature sensor    kt is the estimated body tissue thermal conductivity constant of the tissue,is a common calculation instruction. Depending on the concrete requirements due to special environmental conditions, other calculation instructions are applicable, but they do not change anything in the above-mentioned principle of measurement.
The thermal conductivity constant of human tissue depends significantly on the degree of blood flow. The use of a double temperature sensor presupposes stable thermal conditions with the most constant thermal conductivity possible. This requires, on the one hand, tissue areas with low or uniform blood flow, which are located between the body surface and the body core zone to be measured. A second requirement imposed on the position of the double temperature sensor is that the thinnest possible fat layer or no fat layer shall be located between the sensor and the core zone to be measured, because the strong insulating action of fatty tissue may hinder such measurements of the body core temperature as a whole or make them inaccurate. It is therefore known that a double temperature sensor may be arranged at the head in the area of the top. This position meets the above-mentioned requirements. The double temperature sensor is attached, for example, by means of a head strap.
However, this placement may sometimes be associated with a considerable loss of wearing comfort. Placing the double temperature sensor in the head area is ruled out in other cases because of insufficient space.
U.S. Pat. No. 5,816,706 discloses a heat flow sensor, which comprises, in principle, two parallel double temperature sensors, which are provided for measuring a body temperature near the core. It is assumed in this process that the body tissue has no constant thermal conductivity because of the variable blood flow. Two parameters, namely, the current thermal conductivity constant, which depends on the blood flow, and then, by means of this value, the body core temperature near the core of the body, can be determined simultaneously by means of the process and the double arrangement of the heat flow sensor. This process and the sensor are, however, substantially more complicated than a simple double temperature sensor. An arrangement with a plurality of double temperature sensors is, moreover, substantially larger and displays reduced dynamics during temperature measurements because of its high heat capacity.